A variety of methods and apparatus for three dimensional modelling of articles including prosthetic implants are known. Many of these techniques employ digitised information from CAD-CAM design systems or data captured and/or reconstructed from a variety of reflection and/or transmission scanning devices.
Such scanning devices include laser and acoustic reflection apparatus and various types of transmission apparatus including X-ray, magnetic resonance imaging (MRI) including spiral scan MRI, magnetic resonance angiography (MRA), positron emission (PET or SPECT) as well as ultrasonic radiation. Typically, data is captured by scanning a series of spaced parallel planes which may then be combined by computer tomography (CT) techniques to reconstruct a two or three dimensional projection of the article so scanned.
Modelling of anatomical pathology using computed tomography data is well known for pre-operative planning and rehearsal of procedures and in the manufacture of prosthetic devices.
U.S. Pat. No. 4,436,684 describes a non invasive method for forming prostheses of skeletal structures for use in reconstructive surgery. Three dimensional coordinate data is obtained directly from the digital data generated by the computed tomographic information. The three dimensional coordinate data is then utilised to generate three dimensional cylindrical coordinates which are specified relative to an origin which is coincident with the origin of a coordinate system used in a sculpting tool apparatus to specify the spatial location of a cutting tool relative to a workpiece rotating on a turntable.
Due to difficulties in supporting the workpiece however it is generally not possible to sculpt an entire three dimensional model of an article, rather, this system is employed to construct models of portions of skeletal structures to act as male or female mould surfaces for construction of prosthetic inlays or onlays.
This apparatus and system however cannot construct a hollow model having faithfully reproduced external and internal surfaces and structural features.
U.S. Pat. No. 4,976,737 describes a method of forming a prosthetic device by employing the apparatus and method described in U.S. Pat. No. 4,436,684 to form a template which may be used directly or indirectly to create a mould surface for moulding a polyurethane impregnated Dacron (Trade Mark) prosthesis. This document describes in detail a "mirror imaging" technique to generate digital data for reconstruction of a missing, damaged or deformed portion of a skeletal structure by transferring image data prom one side of an axis of symmetry to another.
Stereolithographic modelling of engineering components from UV sensitive cross-linkable acrylic polymers using CAD/CAM digital data is known. Of more recent times, the use of stereolithography for creation of three dimensional models of bony structures has been reported.
Stereolithographic modelling of anatomical pathology to provide a far more accurate means for physicians and surgeons to examine the condition of a patient for the purposes of diagnosis and for surgical procedures. Rather than rely upon say a solid model representing external features alone (as with U.S. U.S. Pat. No. 4,436,684, with or without two dimensional tomographic images, stereolithographically reproduced representations of anatomical pathology provide an almost exact replica of both internal and external features of a region under consideration.
Moreover, such stereolithographically reproduced models permit surgical procedures to be pre-planned and rehearsed with a great deal of precision to minimise risks and trauma and should enable a means for preparing accurate prostheses for surgical repair of defects or in reconstructive surgery.
One of the difficulties in reconstructing three dimensional co-ordinate data from X-ray tomographic scans is that in order to minimise the amount of radiation to which a patient is exposed, the tomographic "slices" are relatively widely spaced and complex computer programmes are required to reconstruct this scanning data. Typically a "slice" is about 1.5 mm in thickness and "slice" data is obtained at about 1.0 mm intervals. A scan of an adult human skull may thus comprise 70-80 tomographic "slices".
A comparison of three dimensional CT image reconstructions using a destructive mechanical milling process and a constructive stereolithographic modelling process is described in "Paediatric craniofacial surgery: Comparison of milling and stereolithography for 3D model manufacturing", Pediatr. Radiol. (1992) 22: 458-460. This article addresses the limitations of the milling process and concludes that while stereolithography is extremely expensive by comparison, "The slice oriented construction of the model corresponds well with the cross-sectional imaging methods and promisses (sic) for the future a direct transfer from image slice to object slice."
Similar mechanical and stereolithographic modelling processes are described respectively in "Computed-Aided Simulation, Analysis, and Design in Orthopaedic Surgery", Orthopaedic Clinics of North America--Vol 17, No. 4, October 1986 and "Solid models for CT/MR image display: accuracy and utility in surgical planning", /SPIE Vol 1444 Image Capture, Formatting and Display (1991): 2-8.
Both of the references referred to immediately above describe in detail a computed tomography slice processing technique utilising proprietary software to trace all bone boundaries in the image volume after empirically determining the threshold for cortical bone. The algorithm, after exhaustively searching each image, locates the inner and outer edges of cortical bone objects and generates a contour volume data set. This data set is passed to a reformat program to generate the SLA build file containing information necessary to operate the stereolithography apparatus.
In both of these references, the technique requires that the exhaustive contour descriptions must be replicated four times to provide a finished layer of 0.25 mm in thickness. This repetition is necessary to reconstruct the CT axial resolution as one CT slice equals four SLA layers.
In transforming contour data to CAD data, a number of algorithms are available. A simple algorithm uses simple thresholded segmentation to produce voxel faces as paired triangles. A more complex technique uses the "Marching Cubes" algorithm which interpolates slices to yield a surface composed of sub-voxel polygons. The "Marching Cubes" algorithm is described in "Two algorithms for the three-dimensional reconstruction of tomograms", Med Phys. 15(3): 320-7, and "Marching Cubes: a high resolution 3D surface construction algorithm" Computer Graphics. 21:163-169.
A suitable proprietary algorithm which has been used with the method of the present invention is "ANALYZE" three dimensional imaging software (v.2.4) by The Mayo Biomedical Imaging Resources.
U.S. Pat. No. 5,357,429 describes initial imaging of an object at a variety of non perpendicular angles with respect to a longitudinal axis of the object and, after construction and inspection of respective three dimensional images representing each set of parallel two dimensional images, a decision is made to select one set of parallel two dimensional data sets to generated directly the build of a three dimensional model. Accordingly, the upright axis along which the model is built is orthogonal to both the build and scan planes which are parallel to each other.
U.S.Pat. No. 5,452,407 concerns a method for converting image data to vector data wherein the two dimensional image data sets are computed in parallel planes at closer intervals than planes in which the two dimensional cross-sectional images are obtained by scanning.
Neither of U.S. Pat. No. 5,357,429 or U.S. Pat. No. 5,452,407 contemplate utilising computed two dimensional image data sets in planes other than parallel to the original scan planes.
Other prior art references dealing with image reconstruction and/or modelling utilising scan image data are U.S. Pat. Ser. Nos. 5,299,288, 5,554,190, 5,454,383, 5,443,510, 5,373,860, 5,358,935, 4,936,862, 4,902,290, 5,127,037, 5,612,885, 5,487,012, 4,589,992, 4,953,087, 4,976,737, 5,217,653 and 5,231,470. International publications WO,A,9208200, WO,A,8910801 and WO,A, 9106378 as well as European patent application EP,A,574099 describe similar processes.
None of the above referenced processes describe modelling of articles in a plane which is not parallel to the original planes in which scanning occurs.
In order to control the apparatus for constructive modelling, contour information determined from tomograms this may be introduced into a CAD system to generate surface models composed of triangular approximations which is the standard interface between a CAD system and the modelling apparatus.
In addition to contour construction, the region between the inner and outer boundaries must be defined by hatch vectors to enable the solid region to be formed by cross linking of monomer in a predefined region in the monomer bath. By generating not only the contours, but also hatchings with different densities it is possible to produce different structures to represent differing structural densities of a scanned article.
Of more recent times however, there has been reported a more direct technique in "Medical Applications of Rapid Prototyping Techniques" :201-216.
This system addresses both the support generation and interpolation problems of earlier systems and is able to create directly from the CT scans the SLA files of both the model and its support structures in a much shorter time.
While it may be advantageous to utilise direct layer interfacing such that the most accurate directions of the input data in the scanning plane are produced on the most accurate directions of the stereolithography apparatus, the lack of true three dimensional data requires, as with the prior art systems, that the orientation of the part in the stereolithography apparatus should be the same as the orientation during the patient scanning operation.
There are a number of serious disadvantages associated with constructive modelling of articles in the same orientation as conventional patient scanning orientation.
As models are built up from successive 0.25 mm layers of polymerised resin in, say a stereolithographic modelling process, an upright model will take substantially longer to manufacture than a horizontally orientated model. For example, a 50 mm diameter cranial defect would require about 200 layers of polymerised material when in an upright position as against about 10-20 layers when the model is built in a horizontal orientation. Costs of model production could therefore be substantially reduced if manufacturing time could be reduced by selective orientation of models for constructive modelling.
Moreover, selective orientation of models during manufacture would permit a plurality of objects to be simultaneously modelled and orientated in the most efficient manner.
A further disadvantage is that with model construction limited to a single orientation, it is not possible to selectively orientate the model for construction to minimise the extent of support structure which must subsequently be removed from the completed model in say stereolithography, laser sintering, 3D printing or like processes.
In all prosthetic implant surgery it is essential that a very close fit is obtained between the prosthesis and the tissue to which it is attached if an effective bond is to be obtained from tissue growth. Accordingly, there is a need for a much more accurate method for construction of prosthetic implants, both for hard and soft tissue regions, to ensure an initial accurate fit and accurate contour to avoid intraoperative delays while adjustments, contour changes or prolonged attachment procedures are undertaken.
It would also be advantageous in arterial and vascular surgery to provide complex branched prostheses which require attachment to blood vessels at the free ends of the prosthesis rather than having to construct the prostheses from tubular sections of varying diameters intraoperatively as is the case at present.
Similarly in rapid prototyping of industrial or other models, particularly those comprising a plurality of separate partes, it is essential to achieve accurate and cost-effective modelling to reduce development time.